Method and apparatus for calibrating an envelope tracking system

ABSTRACT

A method of calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency (RF) transmitter module of a wireless communication unit. The method includes, within at least one signal processing module of the wireless communication unit: applying training signal having time-variant envelope to input of the RF transmitter module; receiving indication of instantaneous output signal value(s) for the power amplifier module in response to the training signal; and adjusting timing alignment between envelope tracking path of the envelope tracking system and transmit path of the RF transmitter module to align envelope tracking power amplifier module supply voltage to instantaneous envelope(s) of a waveform signal to be amplified by the power amplifier module, based at least partly on the received indication of the instantaneous output signal value(s) for the power amplifier module in response to the training signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.61/700,282, filed on Sep. 12, 2012 and incorporated herein by reference.

BACKGROUND

The field of this invention relates to a method and apparatus forcalibrating an envelope tracking system, and in particular to a methodand apparatus for calibrating an envelope tracking system for a supplyvoltage for a power amplifier module within a radio frequency (RF)transmitter module of a wireless communication unit.

A primary focus and application of the present invention is the field ofradio frequency (RF) power amplifiers capable of use in wirelesstelecommunication applications. Continuing pressure on the limitedspectrum available for radio communication systems is forcing thedevelopment of spectrally-efficient linear modulation schemes. Since theenvelopes of a number of these linear modulation schemes fluctuate,these result in the average power delivered to the antenna beingsignificantly lower than the maximum power, leading to poor efficiencyof the power amplifier. Specifically, in this field, there has been asignificant amount of research effort in developing high efficiencytopologies capable of providing high performances in the ‘back-off’(linear) region of the power amplifier.

Linear modulation schemes require linear amplification of the modulatedsignal in order to minimise undesired out-of-band emissions fromspectral re-growth. However, the active devices used within a typical RFamplifying device are inherently non-linear by nature. Only when a smallportion of the consumed DC power is transformed into RF power, can thetransfer function of the amplifying device be approximated by a straightline, i.e. as in an ideal linear amplifier case. This mode of operationprovides a low efficiency of DC to RF power conversion, which isunacceptable for portable (subscriber) wireless communication units.Furthermore, the low efficiency is also recognised as being problematicfor the base stations.

Additionally, the emphasis in portable (subscriber) equipment is toincrease battery life. To achieve both linearity and efficiency, socalled linearisation techniques are used to improve the linearity of themore efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’amplifiers. A number and variety of linearising techniques exist, whichare often used in designing linear transmitters, such as CartesianFeedback, Feed-forward, and Adaptive Pre-distortion.

Voltages at the output of the linear, e.g. Class AB, amplifier aretypically set by the requirements of the final RF power amplifier (PA)device. Generally, the minimum voltage of the PA is significantly largerthan that required by the output devices of the Class AB amplifier.Hence, they are not the most efficient of amplification techniques. Theefficiency of the transmitter (primarily the PA) is determined by thevoltage across the output devices, as well as any excess voltage acrossany pull-down device components due to the minimum supply voltage (Vmin)requirement of the PA.

In order to increase the bit rate used in transmit uplink communicationchannels, larger constellation modulation schemes, with an amplitudemodulation (AM) component are being investigated and, indeed, becomingrequired. These modulation schemes, such as sixteen-bit quadratureamplitude modulation (16-QAM), require linear PAs and are associatedwith high ‘crest’ factors (i.e. a degree of fluctuation) of themodulation envelope waveform. This is in contrast to the previouslyoften-used constant envelope modulation schemes and can result insignificant reduction in power efficiency and linearity.

To help overcome such efficiency and linearity issues a number ofsolutions have been proposed. One technique known as envelope trackingrelates to modulating the PA supply voltage to match (track) theenvelope of the radio frequency waveform being transmitted by the RF PA.With envelope tracking, the instantaneous PA supply voltage (VPA) of thewireless transmitter is caused to approximately track the instantaneousenvelope (ENV) of the transmitted RF signal. Thus, since the powerdissipation in the PA is proportional to the difference between itssupply voltage and output voltage, envelope tracking enables an increasein PA efficiency, reduced heat dissipation, improved linearity andincreased maximum output power, whilst allowing the PA to produce theintended RF output.

FIG. 1 illustrates a graphical representation 100 of two alternative PAsupply voltage techniques according to the related art; a firsttechnique that provides a fixed supply voltage 105 to a PA, and a secondtechnique whereby the PA supply voltage is modulated to track the RFenvelope waveform 115. In the fixed supply case, excess PA supplyvoltage headroom 110 is used (and thereby potentially wasted),irrespective of the nature of the modulated RF waveform being amplified.However, for example in the PA supply voltage tracking of the RFmodulated envelope case 115, excess PA supply voltage headroom can bereduced 120 by modulating the RF PA supply, thereby enabling the PAsupply to accurately track the instant RF envelope.

The mapping function between ENV and VPA is critical for optimumperformance (efficiency, gain, and adjacent channel power (ACP)). Alsocritical to system performance is timing alignment between the RF signaland VPA at the PA.

Envelope-tracking can be combined with digital pre-distortion (DPD) onthe RF signal to improve ACP robustness. Since the ET system is often amultichip implementation involving function blocks in digital baseband(BB), analogue BB, RF transceiver, power management and PA, consistentET system performance cannot easily be guaranteed across all devices byhardware. There is therefore a need for some level of transceivercalibration in order to accurately map and centre the ET performance ofeach device leaving the production line. To make envelope-tracking acost-effective technology, it is desirable to minimize any extraproduction calibration time and/or use of external characterisationequipment.

Thus, there is a need for an efficient and cost effective solution tothe problem of ET system calibration. In particular, it would thereforebe advantageous for an on-board auto-calibration method that compensatesfor part-to-part variation but preferably does not add any extra testingcosts of significance.

SUMMARY

Accordingly, the invention seeks to mitigate, alleviate or eliminate oneor more of the above mentioned disadvantages singly or in anycombination. Aspects of the invention provide a method and apparatus forcalibrating an envelope tracking system for a supply voltage for a poweramplifier module within a radio frequency, RF, transmitter module of awireless communication unit.

According to a first aspect of the invention, there is provided a methodcomprising, within at least one signal processing module of the wirelesscommunication unit: applying a training signal comprising an envelopethat varies with time to an input of the RF transmitter module;receiving at least an indication of at least one instantaneous outputsignal value for the power amplifier module in response to the trainingsignal; and adjusting a timing alignment between an envelope trackingpath of the envelope tracking system and a transmit path of the RFtransmitter module to align an envelope tracking power amplifier modulesupply voltage to an instantaneous envelope of a waveform signal to beamplified by the power amplifier module, based at least partly on thereceived at least an indication of the at least one instantaneous outputsignal value for the power amplifier module in response to the trainingsignal.

In this manner, an efficient and cost effective solution to the knownproblems of ET system calibration is provided. In addition, the methodand apparatus may be applied to an on-board auto-calibration method thatcompensates for part-to-part variation without significantly incurringany extra testing costs.

According to a second aspect of the invention, there is provided anon-transitory computer program product comprising executable programcode for calibrating an envelope tracking system for a supply voltagefor a power amplifier module within a radio frequency, RF, transmittermodule of a wireless communication unit, the executable program codeoperable for, when executed at a communication unit, performing themethod of the fifth aspect.

According to a third aspect of the invention, there is provided acommunication unit comprising: a radio frequency, RF, transmitter modulecomprising an envelope tracking system for a supply voltage for a poweramplifier module within the RF transmitter module; and at least onesignal processing module for calibrating envelope tracking system. Theat least one signal processing module and arranged to: apply a trainingsignal comprising an envelope that varies with time to an input of theRF transmitter module; receive at least an indication of at least oneinstantaneous output signal value for the power amplifier module inresponse to the training signal; and adjust a timing alignment betweenan envelope tracking path of the envelope tracking system and a transmitpath of the RF transmitter module to align an envelope tracking poweramplifier module supply voltage to at least one instantaneous envelopeof a waveform signal to be amplified by the power amplifier module,based at least partly on the received at least an indication of the atleast one instantaneous output signal value for the power amplifiermodule in response to the training signal.

According to a fourth aspect of the invention, there is provided anintegrated circuit for a communication unit comprising a radiofrequency, RF, transmitter module comprising an envelope tracking systemfor a supply voltage for a power amplifier module within the RFtransmitter module. The integrated circuit comprises at least one signalprocessing module for calibrating the envelope tracking system. The atleast one signal processing module being arranged to: apply a trainingsignal comprising an envelope that varies with time to an input of theRF transmitter module; receive at least an indication of at least oneinstantaneous output signal value for the power amplifier module inresponse to the training signal; and adjust a timing alignment betweenan envelope tracking path of the envelope tracking system and a transmitpath of the RF transmitter module to align an envelope tracking poweramplifier module supply voltage to at least one instantaneous envelopeof a waveform signal to be amplified by the power amplifier module,based at least partly on the received at least an indication of the atleast one instantaneous output signal value for the power amplifiermodule in response to the training signal.

These and other aspects of the invention will be apparent from, andelucidated with reference to, the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, aspects and embodiments of the invention will bedescribed, by way of example only, with reference to the drawings. Inthe drawings, like reference numbers are used to identify like orfunctionally similar elements. Elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.

FIG. 1 illustrates a graphical representation of two alternative PAsupply voltage techniques according to the related art.

FIG. 2 illustrates a simplified block diagram of an example of awireless communication unit.

FIG. 3 illustrates a simplified generic block diagram of an example of apart of an RF transceiver architecture.

FIG. 4 illustrates a simplified flowchart of an example of a method ofcalibrating at least a part of an envelope tracking system within an RFtransceiver.

FIG's 5 and 6 illustrate graphs showing examples of a relationshipbetween voltage supply, input power and output power for a poweramplifier module.

FIG. 7 illustrates a graph showing an example of a relationship betweenvoltage supply, input power and gain for a power amplifier module.

FIG. 8 illustrates graphs showing an example of a training signal.

FIG. 9 illustrates a graph showing an example of a voltage supply for apower amplifier module being de-troughed.

FIG. 10 illustrates a graph showing an example of a relationship betweenpower amplifier gain and voltage supply envelope tracking alignment.

FIG. 11 illustrates a graph showing an example of a relationship betweenpower amplifier added phase and voltage supply envelope trackingalignment.

DETAILED DESCRIPTION

Examples of the invention will be described in terms of one or moreintegrated circuits for use in a wireless communication unit, such asuser equipment in third generation partnership project (3GPP™) parlance.However, it will be appreciated by a skilled artisan that the inventiveconcept herein described may be embodied in any type of integratedcircuit, wireless communication unit or wireless transmitter thatcomprises or forms a part of an envelope tracking system. Furthermore,because the illustrated embodiments of the present invention may for themost part, be implemented using electronic components and circuits knownto those skilled in the art, details will not be explained in anygreater extent than that considered necessary as illustrated below, forthe understanding and appreciation of the underlying concepts of thepresent invention and in order not to obfuscate or distract from theteachings of the present invention.

Referring first to FIG. 2, a block diagram of a wireless communicationunit (sometimes referred to as a mobile subscriber unit (MS) in thecontext of cellular communications or a user equipment (UE) in terms ofa 3^(rd) generation partnership project (3GPP™) communication system) isshown, in accordance with one example embodiment of the invention. Thewireless communication unit 200 contains an antenna 202 preferablycoupled to a duplex filter or antenna switch 204 that provides isolationbetween receive and transmit chains within the wireless communicationunit 200.

The receiver chain 210, as known in the art, includes receiver front-endcircuitry 206 (effectively providing reception, filtering andintermediate or base-band frequency conversion). The front-end circuitry206 is coupled to a signal processing function 208. An output from thesignal processing function 208 is provided to a suitable user interface230, which may encompass a screen or flat panel display. A controller214 maintains overall subscriber unit control and is coupled to thereceiver front-end circuitry 206 and the signal processing function 208(generally realised by a digital signal processor (DSP)). The controller214 is also coupled to a memory device 216 that selectively storesvarious operating regimes, such as decoding/encoding functions,synchronisation patterns, code sequences, and the like.

In accordance with examples of the invention, the memory device 216stores modulation data, and power supply data for use in supply voltagecontrol to track the envelope of the radio frequency waveform to beoutput by the wireless communication unit 200. Furthermore, a timer 218is operably coupled to the controller 214 to control the timing ofoperations (transmission or reception of time-dependent signals and in atransmit sense the time domain variation of the PA supply voltage withinthe wireless communication unit 200).

As regards the transmit chain, this essentially includes the userinterface 230, which may encompass a keypad or touch screen, coupled inseries via signal processing function 228 to transmitter/modulationcircuitry 222. The transmitter/modulation circuitry 222 processes inputsignals for transmission and modulates and up-converts these signals toa radio frequency (RF) signal for amplifying in the power amplifier (PA)module or integrated circuit 224. RF signals amplified by the PA moduleor PA integrated circuit 224 are passed to the antenna 202. Thetransmitter/modulation circuitry 222, power amplifier 224 and PA supplyvoltage module 225 are each operationally responsive to the controller214, with the PA supply voltage module 225 additionally responding to areproduction of the envelope modulated waveform from thetransmitter/modulation circuitry 222.

The signal processor function 228 in the transmit chain may beimplemented as distinct from the processor 208 in the receive chain 210.Alternatively, a single processor may be used to implement processing ofboth transmit and receive signals, as shown in FIG. 2. Clearly, thevarious components within the wireless communication unit 200 can berealised in discrete or integrated component form, with an ultimatestructure therefore being merely an application-specific or designselection.

Furthermore, in accordance with examples of the invention, thetransmitter/modulation circuitry 222, together with power amplifier 224,PA supply voltage 225, memory device 216, timer 218 and controller 214have been adapted to generate a power supply to be applied to the PA224. For example, a power supply is generated that is suitable for awideband linear power amplifier, and configured to track the envelopewaveform applied to the PA 224.

Referring now to FIG. 3, there is illustrated a generic example blockdiagram of a part of an RF transceiver architecture 300 of a wirelesscommunication unit, such as the wireless communication unit 200illustrated in FIG. 2. In the transmit direction, the transceiverarchitecture 300 comprises transmitter/modulation circuitry 222 operablycoupled between a baseband component (denoted as “BB” in FIG. 3) 310,for example residing within the signal processing function 228 and/orthe controller 214 of FIG. 2, and a PA module (denoted as “PA” in FIG.3) 224. The PA module 224 is operably coupled to the antenna (denoted as“ANT” in FIG. 3) 202 via a duplex filter (DPX) and an antenna switchmodule (ASM), illustrated generally at 204. A PA supply voltagemodulator (denoted as “MOS” in FIG. 3) 320 is arranged to modulate thesupply voltage to the PA module 224 in accordance with a signal receivedfrom a further baseband component (denoted as “BB” in FIG. 3) 312, forexample residing within the controller 214 of FIG. 2, via adigital-to-analogue converter (DAC) 330. In this manner, the PA supplyvoltage modulator 320, DAC 330 and corresponding baseband component 312may be configured to perform envelope tracking modulation of the supplyvoltage provided to the PA module 224 such that the supply voltageprovided to the PA module 224 substantially tracks an envelope of a RFwaveform being amplified by the PA module 224. Accordingly, the PAsupply voltage modulator 320, DAC 330 and corresponding basebandcomponent 312 may form (at least a part of) an envelope tracking system340 of the transceiver architecture 300. In the receive direction, thetransceiver architecture 300 comprises receiver front-end circuitry(denoted as “RX” in FIG. 3) 206 operably coupled between the duplexfilter and a further baseband component (denoted as “BB” in FIG. 3) 314,for example residing in the signal processing function 208 and/or thecontroller 214 of FIG. 2.

As previously mentioned, the mapping function between the envelope ofthe RF waveform being amplified and the modulation of the PA supplyvoltage is critical for optimum performance (efficiency, gain, andadjacent channel power (ACP)). Also critical to system performance istiming alignment between the RF signal and VPA at the PA. To this end,in the illustrated example, the transceiver architecture 300 illustratedin FIG. 3 further comprises a detection component (denoted as “DET” inFIG. 3) 350 arranged to receive an indication of an output of the PAmodule 224, and to enable the detection of the mapping and alignment ofthe envelope tracking system, as described in greater detail below.

In the illustrated example, the detection component 350 is illustratedas comprising a discrete component within the transceiver architecture300, arranged to receive an indication of an output signal of the PAmodule 224, and to output an indication of a detected output power ofthe PA module 224 to a baseband component (denoted as “BB” in FIG. 3)316, for example residing within the controller 214 of FIG. 2. Thedetection component 350 may comprise functionality such asamplification, down-mixing, analogue-to-digital conversion, etc. In theillustrated example, the detection component 350 is operably coupled toan antenna coupler (CPL) 360, and arranged to receive an indication ofthe output signal of the PA module 224 in the form of the RF signalprovided to the antenna 202. Advantageously, by using the RF signalprovided to the antenna 202 as the indication of the output signal ofthe PA module 224 in this manner, variations within the duplex filterand antenna switch module 204 may also be compensated for during anycalibration subsequently performed based on the detected output powersignal generated by the detection component 350.

It will be apparent that the present invention is not limited to thespecific example transceiver architecture 300 illustrated in FIG. 3, andmay equally be applied to other transceiver architectures. For example,in some alternative architectures the detection component 350 may beoperably coupled directly to the output of the PA module 224 andarranged to receive an indication of the output signal of the PA module224 substantially directly. In some further alternative architecturesthe detection component 350 may be at least partially merged within thereceiver front-end circuitry 206, and arranged to receive an indicationof the output signal of the PA module 224 via the duplex filter. In thismanner, the detection component 350 could re-use at least some of thefunctionality of the receiver front-end circuitry 206 such as ADCs,baseband functionality, etc.

Referring now to FIG. 4, there is illustrated a simplified flowchart 400of an example of a method of calibrating at least a part of an envelopetracking system within an RF transceiver, such as the envelope trackingsystem 340 of FIG. 3. In the example illustrated in FIG. 3, the methodof FIG. 4 may be implemented within one or more of the basebandcomponents 310, 312, 314, 316.

The method starts at step 405, and moves on to step 410 whereconventional fixed-drain calibration of a transmit chain of the RFtransceiver is performed in order to calibrate the PA and analoguetransmit gain steps. Such fixed-drain calibration may comprise, forexample, a first step whereby the detection feedback path (CPL to DETpath 355 in FIG. 3) is calibrated using at least one measurementobtained by way of an external power meter 370. Once the detectionfeedback path 355 has been calibrated, and it provides accurate powermeasurements, a lookup table (LUT) may be created containing, for eachdesired output power range, corresponding baseband,transmitter/modulation circuitry and PA gain settings. In addition, alookup table (LUT) within the baseband component 312 of the envelopetracking system 340 for storing constant PA supply voltage values andtheir respective PA output power values may be populated using thedetection component 350.

Note that the PA supply voltage value entries derived at this stage ofthe illustrated example during fixed drain calibration will be based ona fixed PA supply voltage within the RF transceiver (as opposed to aninstantaneous envelope tracking PA supply voltage).

Having performed the fixed-drain calibration, the next stage in themethod illustrated in FIG. 4 comprises performing an initial (coarse)calibration of the envelope tracking system. This coarse calibrationstage begins at step 415 where the baseband component(s) is/areconfigured to produce a continuous wave to be output by the transmitchain of the transceiver, and the envelope tracking path is configuredto operate in a characterisation mode. For example the basebandcomponent(s) may be configured to produce the continuous wave:

z(t)=A·exp(jω ₀ t)  [Equation 1]

If desired, the continuous wave may be duty cycled to reduce the averagepower and have thermal conditions closer to the conditions in the field.The envelope tracking path may be considered to be in a characterisationmode when the PA supply voltage is not derived from the envelope of thewaveform signal to be amplified, but set to a reference voltage(VPA_ref). VPA_ref may be a platform-dependent, predefined voltagechosen based on lab characterization or datasheet data of the particularPA being used in the system. The actual VPA_ref voltage at the PA supplywill typically vary from part to part due to component variations withinthe supply voltage path (e.g. within the PA supply voltage modulator320, DAC 330 and corresponding baseband component 312 in the exampleillustrated in FIG. 3.). However, the proposed calibration method hereindescribed is tolerant of such variations, as will become apparent.

Having configured the baseband component(s) to produce a continuous waveand the envelope tracking path to operate in a characterisation mode(i.e. with the PA supply voltage VPA set to the constant referencevoltage (VPA_ref)), the method moves on to step 420, where referencedata point values are determined for an upper limit of the ‘back-off’(linear) operating region of the PA module, where the PA module is mostefficient. Accordingly, in the illustrated example a reference inputpower signal (Pin_ref) to the PA module is found that generates apredefined reference output power signal (Pout_ref) when the PA supplyvoltage is set to the constant reference voltage (VPA_ref). Inparticular, the reference voltage (VPA_ref) and the predefined referenceoutput power signal (Pout_ref) are chosen such that the PA is biasedtowards the upper limit of the back-off region of operation.

In the illustrated example, this is an iterative process where, for theexample illustrated in FIG. 3, the output power of the PA module 224 isdetected by the detection component 350 and corresponding basebandcomponent 316, and at least an indication thereof may be fed back to thebaseband component 310 responsible for outputting the RF waveform to thePA module 224 via the transmitter/modulation circuitry 222. In thismanner, the baseband component 310 may iteratively adjust the inputpower of the RF waveform signal provided to the PA module 224 via thetransmitter/modulation circuitry 222, in response to output powerindications received from the detection feedback path comprising thedetection component 350 and corresponding baseband component 316, untilthe predefined reference output power signal (Pout_ref) is reached. Forexample, the input power (Pin) may be iteratively adjusted based onEquation 2 below:

Pin(k)=Pin(k−1)+(Pout_ref−Pout(k−1))  [Equation 2]

where power is expressed in dBm. Alternatively Equation 2 could beexpressed in mW, or other units, and the iterative adjustment doneaccordingly. The predefined values for the constant reference voltage(VPA_ref) and the reference output power signal (Pout_ref) may be chosenin accordance with the particular PA module, duplex filter and antennaswitch module used.

For example, and referring to FIG. 5 for illustration, a predeterminedPA supply voltage (VPA) is set to 3.5V, and desired reference outputpower (Pout_ref) is set to 27 dBm. For an initial input power Pin(0)level, an output power (Pout) of 28 dBm is detected. The input power Pinmay then be iteratively adjusted (e.g. reduced) until the desired outputpower of 27 dBm is achieved.

In case a duty cycled continuous wave input signal is used (e.g. toreduce the average power and have thermal conditions closer to theconditions in the field), the target output power would comprisePout_ref*duty_cycle.

As mentioned above, the reference voltage (VPA_ref) and the predefinedreference output power signal (Pout_ref) are chosen such that the PA isbiased towards the top end of the ‘back-off’ (linear) region ofoperation, in order to maximise PA efficiency. However, because ofmanufacturing tolerances of the various individual components, theactual VPA_ref voltage at the PA supply will typically vary from part topart, and it is difficult to accurately predict where the upper limit ofthe back-region for each individual PA module will exist. Accordingly,in the illustrated example, having determined the reference data pointvalues for the upper limit of the ‘back-off’ (linear) operating regionof the PA module, comprising Pin=Pin_ref, Pout=Pout_ref and VPA=VPA_ref,the method moves on to step 425, where the input power for the PA ismaintained at Pin_ref, and a PA supply voltage (VPA_cmp_a) is found thatproduces an output power of the PA module equal to Pout_ref reduced by apredefined gain compression factor ΔG, such that:

Pout=Pout_ref−ΔG  [Equation 3]

where power is expressed in dBm and ΔG in dB. Equivalent implementationsof Equation 3 are possible in other units like mW or W. As illustratedin FIG. 4, this may also comprise an iterative process. For example, thePA supply voltage VPA may be iteratively adjusted based on Equation 4below:

$\begin{matrix}{{{VPA}(k)} = {{{VPA}\left( {k - 1} \right)} + {\left( {{Pout\_ tgt} - {{Pout}\left( {k - 1} \right)}} \right) \cdot \frac{\delta \; V}{\delta \; P}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

where Pout_tgt=Pout_ref−ΔG, and

$\frac{\delta \; V}{\delta \; P}$

can be adjusted at each iteration based on the previous iteration. FIG.6 illustrates at 610 an example in which an input power of Pin_ref ismaintained, and a PA supply voltage (VPA_cmp_a) is found that producesan output power of the PA module equal to Pout_ref (27 dBm) reduced by apredefined gain compression factor ΔG of 1 dB; i.e. that produces anoutput power Pout=26 dBm. Note that the Pout values in Equations 4 couldalso be expressed in mW or W instead of dBm.

By reducing the voltage power supply to the PA module in this manner toachieve an output power reduced by the gain compression factor AG, thePA module is driven more forcefully, allowing it to drop down into itscompressed region by a small amount (dependent on the size of the gaincompression factor ΔG. As a result, variations in where the upper limitof the back-region for individual PA modules due to manufacturingtolerances may be tolerated. Thus, a ‘tolerant’ upper limit PA supplyvoltage (VPA_cmp_a) may be found that achieves a high end output power(Pout_ref−ΔG) for a given reference input power (Pin_ref).Significantly, under such operating conditions, an efficient gain forthe PA module is achieved. This method can be used to ensure that allPAs in several wireless communication units operate at the same level ofcompression, regardless of their actual absolute gain.

The gain compression factor AG may be chosen based on any suitablefactors. For example, if GPA_ref−ΔG+Pin_max is less than the maximumrequired peak output power, where GPA_ref represents the PA module gainwhen Pin=Pin_ref, Pout=Pout_ref and VPA=VPA_ref, then ΔG may be deemedtoo large.

Additionally and/or alternatively, if for the minimum input power forwhich the envelope is not de-troughed (as described below) the gain issmaller than GPA_ref−ΔG, and the PA supply voltage VPA is equal toVPA_max, then ΔG may be deemed too small.

Additionally and/or alternatively, if for the minimum input power forwhich the envelope is not de-troughed the gain is bigger thanGPA_ref−ΔG, and VPA is equal to VPA_min, then ΔG may be deemed toolarge.

Additionally and/or alternatively, the gain compression factor ΔG couldbe decided according to the transmitter/modulation circuitry, PA module,duplex filter and antenna switch module (ASM) of the application athand. This would mean that the part-to-part PA gain variation would notbe compensated for. However, this would ensure that all devices operateat the same level of gain compression. This guarantees less part-to-partlinearity performance variation and similar performance degradation dueto temperature changes. Additionally/alternatively, the gain compressionfactor AG could be adjusted depending on the PA module alone. This wouldcompensate for part-to-part PA gain variation, but then differentdevices would operate at different levels of compression.

The values VPA=VPA_cmp_a, Pin=Pin_ref and Pout=Pout_ref−ΔG may then beused to define a first calibration data point, as illustrated at step430.

Having found VPA_cmp_a such that Pout=Pout_ref−ΔG for Pin_ref, anddefined the first calibration data point, at least one furthercalibration data point is required to be derived for the initial(coarse) calibration of the envelope tracking system. Accordingly, atstep 435 the input power for the PA module is reduced by a predefinedamount ΔP, such that (Pin and Pin_ref in dBm, ΔP in dB):

Pin=Pin_ref−ΔP  [Equation 5]

and a PA supply voltage (VPA_cmp_b) is found that produces an outputpower of the PA module equal to Pout_ref reduced by the predefined gaincompression factor ΔG and the predefined amount ΔP, such that:

Pout=Pout_ref−ΔG−ΔP  [Equation 6]

As illustrated in FIG. 4, this may comprise an iterative process. Forexample, the PA supply voltage VPA may be iteratively adjusted based onEquation 4 above, where Pout_tgt=Pout_ref−ΔG−ΔP, and

$\frac{\delta \; V}{\delta \; P}$

can be adjusted at each iteration based on the previous iteration. FIG.6 illustrates at 620 an example in which the input power Pin is setequal to Pin_ref reduced by the predefined amount ΔP of 2 dB, and a PAsupply voltage (VPA_cmp_b) is found that produces an output power of thePA module equal to Pout_ref (27 dBm) reduced by the predefined gaincompression factor ΔG of 1 dB and by the predefined amount ΔP of 2 dB;i.e. that produces an output power Pout=24 dBm. Note that Equations 5and 6 could also be expressed in mW, W or any other unit.

In this manner, a PA supply voltage (VPA_cmp_b) is found that maintainsa substantially constant gain (GPA_ref−ΔG) for the PA module for thereduced output power Pout=Pout_ref−ΔG−ΔP. Thus, in example embodiments,a mapping function between an instantaneous envelope of a waveformsignal to be amplified by the power amplifier module and the poweramplifier module supply voltage may be adjusted to achieve asubstantially constant power amplifier module gain. In the context ofexamples of the invention, a substantially constant power amplifiermodule gain encompasses power amplifier module gain values that arewithin reasonable and acceptable design/engineering tolerances of aparticular implementation.

The values VPA=VPA_cmp_b, Pin=Pin_ref−ΔP and Pout=Pout_ref−ΔG−ΔP maythen be used to define a further (e.g. second) calibration data point,as illustrated at step 440. The initial, coarse calibration of theenvelope tracking system may then be performed based at least partly onthe derived data points, for example comprising population of anenvelope tracking VPA lookup table as illustrated at step 445. Suchcalibration may comprise, say, linear interpolation of the derived datapoints to define a linear VPA mapping profile. Alternatively suchcalibration may comprise using the derived data points to offset and/orscale a pre-characterised VPA mapping profile for a give PA module partnumber.

As previously mentioned, the mapping function between the instantaneousenvelope of the transmitted RF signal and the PA supply voltage iscritical for optimum performance (efficiency, gain, and adjacent channelpower (ACP)). In the example graph showing gain with respect to PAsupply voltage VPA and PA input power Pin illustrated in FIG. 7, inwhich the gain of the PA module is represented by the contours of theillustrated graph, a linearly interpolated mapping function such asdescribed above for the coarse calibration would assume a gaincomprising a straight line passing through the data points for VPA_cmp_a710 and VPA_cmp_b 720.

However, as illustrated in FIG. 7 the actual gain of the PA module isnot perfectly linear. The waveform trajectory is not exactly on top of aconstant gain contour of 27.4 dB, albeit very close. Accordingly, havingperformed the coarse calibration of the envelope tracking system, thenext stage in the method illustrated in FIG. 4 comprises fine-tuning ofthe envelope tracking system calibration. This calibration fine-tuningstage begins at step 450 where the baseband component(s) is/areconfigured to produce an envelope modulated waveform signal to be outputby the transmit chain of the transceiver, and the envelope tracking pathis configured to operate in a tracking mode whereby the PA supplyvoltage VPA is configured based on the mapping function between theinstantaneous envelope of the envelope modulated waveform signal and thePA supply voltage VPA. In the illustrated example, the mapping functionbetween the instantaneous envelope of the envelope modulated waveformsignal and the PA supply voltage VPA is defined within the previouslymentioned lookup table populated in step 445 whereby Pin_ref maps toVPA_cmp_a and Pin_ref−ΔP maps to VPA_cmp_b.

The envelope modulated waveform signal may be based on any suitabletraining signal. For example, the training signal may be defined suchthat it comprises a bandwidth less than the anticipated data bandwidthfor the particular transceiver application, and/or such that itcomprises a peak to average power ratio equivalent to that of a liveuplink modulation for the particular transceiver application. The RMS(root mean square) of the output power may be such that the system ischaracterised within the wanted window of output powers. An example of atraining signal is illustrated in FIG. 8, and defined in Equation 7below:

z(t)=0.5(1+sin(ω₁ t))exp(jω ₂ t)  [Equation 7]

with a peak to average power ratio of I/Q signals (PAR_IQ)=7 dB, and apeak to average power ratio of RF envelope (PAPR_RF)=4.3 dB.

In addition, any envelope modulation settings required according to theVPA signal characteristics may be configured, for example such as DC(direct current) value, AC (alternating current) swing, etc.

Next, at step 455, instantaneous output values for the PA module arecaptured, including information such as power and phase of theinstantaneous output signal, for example via the detection component 350in the example illustrate in FIG. 3. Instantaneous gain values are thencalculated for the PA module in response to the training signal, at step460. For example, the captured output signal may be aligned andde-rotated to allow comparison with the input training signal. In theillustrated example, the mapping function between the instantaneousenvelope of the modulated waveform signal and the PA supply voltage VPA(e.g. the lookup table populated in step 445) is then adjusted, at 465,in order to achieve a substantially constant gain.

As illustrated in FIG. 4, this may also comprise an iterative process.For example, the PA supply voltage VPA may be iteratively adjusted basedon Equation 8 below:

$\begin{matrix}{{{VPA}(k)} = {{{VPA}\left( {k - 1} \right)} + {\left( {{Gain\_ tgt} - {{Gain}\left( {k - 1} \right)}} \right) \cdot \frac{\delta \; V}{\delta \; G}}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

where Gain_tgt is the target constant gain in dB (e.g. GPA_ref−ΔG), and

$\frac{\delta \; V}{\delta \; G}$

can be adjusted at each iteration based on the previous iteration. Anequivalent formula to Equation 8 based on power measurements rather thangain measurements can be used instead if deemed more convenient. Also anequivalent formulation of Equation 8 with the gain expressed in thelinear domain rather than in dB is possible.

In order to avoid very low PA supply voltage VPA levels, and/or to limitthe PA supply voltage VPA AC swing, the PA supply voltage VPA may bede-troughed within the mapping function between the instantaneousenvelope of the transmitted RF signal and the PA supply voltage, forexample as illustrated at 910 in FIG. 9 whereby the PA supply voltageVPA is restricted within the mapping function between the instantaneousenvelope of the transmitted RF signal and the PA supply voltage to aminimum value.

As also mentioned above, timing alignment between the instantaneousenvelope of the transmitted RF signal and the PA supply voltagemodulation is critical to system performance. Initially, the envelopetracking path may be configured to comprise a default timing alignmentwith respect to the modulated waveform input signal at step 450. If thePA supply voltage VPA and the instantaneous envelope of the transmittedRF signal are perfectly aligned, such as illustrated in FIG. 9, theinstantaneous gain of the PA at the point of entering a trough,indicated generally at 920, should be the same as the instantaneous gainof the PA at the point of leaving the trough, indicated generally at930.

However, and as illustrated in FIG. 10, if the PA supply voltage VPA islagging with respect to the instantaneous envelope of the transmitted RFsignal, the PA supply voltage VPA will be too high at the point ofentering the trough, and too low at the point of leaving the trough. Asa result, the instantaneous gain of the PA module at the point ofentering the trough will be too high, whilst the instantaneous gain ofthe PA module at the point of leaving the trough 30 will be too low.Conversely, if the PA supply voltage VPA is leading with respect to theinstantaneous envelope of the transmitted RF signal, the PA supplyvoltage VPA will be too low at the point of entering the trough, and toohigh at the point of leaving the trough. As a result, the instantaneousgain of the PA module at the point of entering the trough will be toolow, whilst the instantaneous gain of the PA module at the point ofleaving the trough will be too high.

Based on this gain symmetry, a timing alignment setting between theenvelope tracking path of the envelope tracking system and a transmitpath of the RF transmitter module may be iteratively updated based onEquation 9 below:

$\begin{matrix}{{{Del}(k)} = {{{Del}\left( {k - 1} \right)} + {\left( {{{GPA\_ in}\left( {k - 1} \right)} - {{GPA\_ out}\left( {k - 1} \right)}} \right) \cdot \frac{\delta \; D}{\delta \; G}}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

where GPA_in is the instantaneous gain of the PA module at the point ofentering the trough, GPA_out is the instantaneous gain of the PA moduleat the point of leaving the trough, and

$\frac{\delta \; D}{\delta \; G}$

can be adjusted at each iteration based on the previous iteration.

As illustrated in FIG. 11, which illustrates the phase added to thesignal in Equation 7 by the PA as a function of time, the added phaseresponse for the output PA training signal shows a similar symmetry tothe gain of the PA supply voltage VPA. Thus, additionally/alternatively,for example at the same time that the magnitude of the gain iscalculated, the added phase of the output PA training signal may also becalculated, and based on this phase symmetry the alignment between thePA supply voltage VPA and the instantaneous envelope of the transmittedRF signal may be iteratively updated based on Equation 10 below:

$\begin{matrix}{{{Del}(k)} = {{{Del}\left( {k - 1} \right)} + {\left( {{{Phase\_ in}\left( {k - 1} \right)} - {{Phase\_ out}\left( {k - 1} \right)}} \right) \cdot \frac{\delta \; P}{\delta \; G}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

where Phase_in is the added phase of the output PA training signal atthe point of entering the trough, Phase_out is the added phase of theoutput PA training signal at the point of leaving the trough, and

$\frac{\delta \; P}{\delta \; G}$

can be adjusted at each iteration based on the previous iteration.

Accordingly, and referring back to FIG. 4, in the illustrated example,having adjusted the mapping function between the instantaneous envelopeof the modulated waveform signal and the PA supply voltage in order toachieve a substantially constant gain, at step 465, the method moves onto step 470 where instantaneous gain values for the power amplifiermodule and/or the added phase of the output PA training signal at troughentry and exit points are calculated. The alignment of the envelopetracking path of the envelope tracking system within the transmit pathis then adjusted based on a symmetry of the calculated instantaneousgain values for the power amplifier module and/or the added phase of theoutput PA training signal, to align the envelope tracking PA supplyvoltage VPA to the instantaneous envelope of a waveform signal to beamplified by the PA module.

As a by-product of the envelope to PA supply voltage mapping functionand alignment characterisation, the AM2AM (amplitude modulation toamplitude modulation) and AM2PM (amplitude modulation to phasemodulation) responses for the transmit chain of the RF transceiverarchitecture are available. If the perfect constant gain VPA mapping hasbeen achieved, the AM2AM response will be ideal (linear). However,achieving such a constant gain VPA mapping does not guarantee an ideal(constant) AM2PM response. In some example embodiments, it iscontemplated that the AM2PM response may be used for digitalpre-distortion (DPD) of the RF signal, assuming there is sufficientbandwidth in the forward path. For example, digital pre-distortion maybe applied to waveform signals prior to being provided to the input ofthe PA module, e.g. within the baseband component 310 in the exampleillustrated in FIG. 3. In some examples, it is contemplated that AM2PMpre-distortion may be applied in this manner, and may be applied to thetraining signal used during the fine tuning of the mapping functioncalibration and alignment steps of the method of FIG. 4. For example, adefault AM2PM pre-distortion may initially be configured and applied atstep 450. The AM2PM pre-distortion may then be refined at each iterationof steps 465 and 475. In this manner, once the constant gain mapping hasbe achieved, and the optimum timing alignment setting found, both theAM2PM response and the AM2PM response may be substantially ideal.

In some examples, for example depending on characteristics of the PAsupply voltage modulator 320 in the example illustrated in FIG. 3 and/oron the intended application for the RF transmitter module 300, thecalibration and alignment steps 455 to 475 in the example methodillustrated in FIG. 4 may need to be repeated for different output powerranges of the PA module, which require different VPA signal levels.Accordingly, if it is determined that these steps need repeating, atstep 480, the method loops back to step 455. Otherwise the method endsat step 485.

In the foregoing specification, the invention has been described withreference to specific examples of embodiments of the invention. It will,however, be evident that various modifications and changes may be madetherein without departing from the broader spirit and scope of theinvention as set forth in the appended claims.

Those skilled in the art will recognize that the boundaries betweenlogic blocks are merely illustrative and that alternative embodimentsmay merge logic blocks or circuit elements or impose an alternatedecomposition of functionality upon various logic blocks or circuitelements. Thus, it is to be understood that the architectures depictedherein are merely exemplary, and that in fact many other architecturescan be implemented which achieve the same functionality.

Any arrangement of components to achieve the same functionality iseffectively ‘associated’ such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as ‘associated with’ each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermediary components. Likewise, any two componentsso associated can also be viewed as being ‘operably connected’, or‘operably coupled’, to each other to achieve the desired functionality.

Furthermore, those skilled in the art will recognize that boundariesbetween the above described operations merely illustrative. The multipleoperations may be combined into a single operation, a single operationmay be distributed in additional operations and operations may beexecuted at least partially overlapping in time. Moreover, alternativeembodiments may include multiple instances of a particular operation,and the order of operations may be altered in various other embodiments.

Also for example, the various components/modules, or portions thereof,may implemented as soft or code representations of physical circuitry orof logical representations convertible into physical circuitry, such asin a hardware description language of any appropriate type.

Also, the invention is not limited to physical devices or unitsimplemented in non-programmable hardware but can also be applied inprogrammable devices or units able to perform the desired devicefunctions by operating in accordance with suitable program code, such asmainframes, minicomputers, servers, workstations, personal computers,notepads, personal digital assistants, electronic games, automotive andother embedded systems, cell phones and various other wireless devices,commonly denoted in this application as ‘computer systems’.

However, other modifications, variations and alternatives are alsopossible. The specifications and drawings are, accordingly, to beregarded in an illustrative rather than in a restrictive sense.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word ‘comprising’ does notexclude the presence of other elements or steps then those listed in aclaim. Furthermore, the terms ‘a’ or ‘an’, as used herein, are definedas one or more than one. Also, the use of introductory phrases such as‘at least one’ and ‘one or more’ in the claims should not be construedto imply that the introduction of another claim element by theindefinite articles ‘a’ or ‘an’ limits any particular claim containingsuch introduced claim element to inventions containing only one suchelement, even when the same claim includes the introductory phrases ‘oneor more’ or ‘at least one’ and indefinite articles such as ‘a’ or ‘an’.The same holds true for the use of definite articles. Unless statedotherwise, terms such as ‘first’ and ‘second’ are used to arbitrarilydistinguish between the elements such terms describe. Thus, these termsare not necessarily intended to indicate temporal or otherprioritization of such elements. The mere fact that certain measures arerecited in mutually different claims does not indicate that acombination of these measures cannot be used to advantage.

What is claimed is:
 1. A method of calibrating an envelope trackingsystem for a supply voltage for a power amplifier module within a radiofrequency (RF) transmitter module of a wireless communication unit; themethod comprising, within at least one signal processing module of thewireless communication unit: applying a training signal comprising anenvelope that varies with time to an input of the RF transmitter module;receiving at least an indication of at least one instantaneous outputsignal value for the power amplifier module in response to the trainingsignal; and adjusting a timing alignment between an envelope trackingpath of the envelope tracking system and a transmit path of the RFtransmitter module to align an envelope tracking power amplifier modulesupply voltage to at least one instantaneous envelope of a waveformsignal to be amplified by the power amplifier module, based at leastpartly on the received at least an indication of the at least oneinstantaneous output signal value for the power amplifier module inresponse to the training signal.
 2. The method of claim 1, wherein themethod comprises: calculating an instantaneous gain of the poweramplifier module at the point of entering a trough of an instantaneousenvelope of the training signal; calculating an instantaneous gain ofthe power amplifier module at the point of leaving a trough of theinstantaneous envelope of the training signal; and adjusting the timingalignment between the envelope tracking path of the envelope trackingsystem and a transmit path of the RF transmitter module to align theenvelope tracking power amplifier module supply voltage to theinstantaneous envelope of a waveform signal to be amplified based atleast partly on a gain symmetry between the instantaneous gain of thepower amplifier module at the point of entering a trough of theinstantaneous envelope of the training signal and the instantaneous gainof the power amplifier module at the point of leaving a trough of theinstantaneous envelope of the training signal.
 3. The method of claim 2,wherein the method comprises adjusting the timing alignment between theenvelope tracking path of the envelope tracking system and a transmitpath of the RF transmitter module to align the envelope tracking poweramplifier module supply voltage to the instantaneous envelope of awaveform signal to be amplified based at least partly on:${{Del}(k)} = {{{Del}\left( {k - 1} \right)} + {\left( {{{GPA\_ in}\left( {k - 1} \right)} - {{GPA\_ out}\left( {k - 1} \right)}} \right) \cdot {\frac{\delta \; D}{\delta \; G}.}}}$4. The method of claim 1, wherein the method comprises: calculating anadded phase of an output signal of the power amplifier at the point ofentering a trough of an instantaneous envelope of the training signal;calculating a phase of the power amplifier output signal at the point ofleaving a trough of an instantaneous envelope of the training signal;and adjusting the timing alignment between the envelope tracking path ofthe envelope tracking system and a transmit path of the RF transmittermodule to align the envelope tracking power amplifier module supplyvoltage to the instantaneous envelope of a waveform signal to beamplified based at least partly on a phase symmetry between the poweramplifier output signal at the point of entering a trough of theinstantaneous envelope of the training signal and the power amplifieroutput signal at the point of leaving a trough of the instantaneousenvelope of the training signal.
 5. The method of claim 4, wherein themethod comprises adjusting the timing alignment between the envelopetracking path of the envelope tracking system and a transmit path of theRF transmitter module to align the envelope tracking power amplifiermodule supply voltage to the instantaneous envelope of a waveform signalto be amplified based at least partly on:${{Del}(k)} = {{{Del}\left( {k - 1} \right)} + {\left( {{{Phase\_ in}\left( {k - 1} \right)} - {{Phase\_ out}\left( {k - 1} \right)}} \right) \cdot {\frac{\delta \; P}{\delta \; G}.}}}$6. The method of claim 1, wherein the method comprises iteratively:receiving at least an indication of instantaneous output signal valuesfor the power amplifier module in response to the training signal; andadjusting the timing alignment between the envelope tracking path of theenvelope tracking system and a transmit path of the RF transmittermodule to align an envelope tracking power amplifier module supplyvoltage to an instantaneous envelope of a waveform signal to beamplified, based at least partly on the received at least an indicationof instantaneous output signal values for the power amplifier module inresponse to the training signal.
 7. The method of claim 1, furthercomprising preceding steps of: setting an envelope tracking path of thetransmitter module into a characterisation mode in which the poweramplifier module supply voltage is not dependent on the at least oneinstantaneous envelope of a received waveform signal; applying acontinuous wave training signal comprising a constant envelope to theinput of the RF transmitter module; deriving a first calibration datapoint for the power amplifier module for which the power amplifiermodule comprises a first gain; deriving at least one further calibrationdata point for the power amplifier module for which the power amplifiermodule comprises the first gain; and calibrating the mapping functionbetween the at least one instantaneous envelope of a waveform signal tobe amplified and the power amplifier module supply voltage based atleast partly on the first and at least one further calibration datapoints.
 8. The method of claim 1, wherein calibrating the mappingfunction between the at least one instantaneous envelope of a waveformsignal to be amplified and the power amplifier module supply voltagebased at least partly on the first and at least one further calibrationdata points comprises performing linear interpolation of the deriveddata points to define a linear mapping profile.
 9. The method of claim1, wherein calibrating the mapping function between the at least oneinstantaneous envelope of a waveform signal to be amplified and thepower amplifier module supply voltage based at least partly on the firstand at least one further calibration data point comprises offsettingand/or scaling a pre-characterised mapping profile.
 10. The method ofclaim 1, wherein deriving the first calibration data point comprises:setting an input power for the power amplifier module to a firstcalibration data point input power; finding a first calibration datapoint power amplifier module supply voltage that produces an outputpower for the power amplifier module equal to a first calibration datapoint output power; and derive the first calibration data point based atleast partly on the first calibration data point input power and thefirst calibration data point power amplifier module supply voltage. 11.The method of claim 4, wherein the method further comprises: setting areference power amplifier module supply voltage; finding a referenceinput power for the power amplifier module that produces an output powerfor the power amplifier module equal to a reference output power,wherein the reference power amplifier module supply voltage and thereference output power are chosen such that the power amplifier moduleis biased towards an upper limit of a back-off region of operation; andconfiguring the first calibration data point input power equal to thereference input power, and the first calibration data point output powerequal to the reference output power less a gain compression factor ΔG.12. The method of claim 5, wherein the gain compression factor AG ischosen based at least partly on at least one of: GPA_ref−ΔG+Pin_max≧amaximum required peak output power, where GPA_ref represents a poweramplifier module gain when Pin=Pin_ref, Pout=Pout_ref and VPA=VPA_ref;for a minimum input power for which VPA is not de-troughed and the PAsupply voltage VPA is equal to VPA_max, gain GPA_ref−ΔG; for a minimuminput power for which VPA is not de-troughed and VPA is equal toVPA_min, gain≦GPA_ref−ΔG; and at least one of the transmitter/modulationcircuitry, PA module, duplex filter and antenna switch module of theapplication at hand.
 13. The method of claim 5, wherein deriving thesecond calibration data point comprises: setting an input power for thepower amplifier module to a second calibration data point input power,the second calibration data point input power being equal to the firstcalibration data point input power reduced by a predefined amount ΔP;finding a second calibration data point power amplifier module supplyvoltage that produces an output power for the power amplifier moduleequal to a second calibration data point output power, the secondcalibration data point output power being equal to the first calibrationdata point output power reduced by the predefined amount ΔP; andderiving the second calibration data point based at least partly on thesecond calibration data point input power and the second calibrationdata point power amplifier module supply voltage.
 14. The method ofclaim 1, wherein the method further comprises, having calibrated themapping function between the at least one instantaneous envelope of awaveform signal to be amplified and the power amplifier module supplyvoltage based at least partly on the first and at least one furthercalibration data points with envelope track path set to acharacterisation, setting the envelope tracking path of the transmittermodule into an envelope tracking mode; the envelope tracking mode beingbased on the calibrated mapping function between the at least oneinstantaneous envelope of a received waveform signal and the poweramplifier module supply voltage.
 15. The method of claim 1, wherein thetraining signal comprising an envelope that varies with time to an inputof the RF transmitter module is defined according to at least one of abandwidth of the training signal being less than an anticipatedbandwidth for an intended application for the RF transmitter module; thetraining signal comprises a peak to average power ratio equivalent tothat of a live uplink modulation for an intended application for the RFtransmitter module; a root mean square of an output power of the poweramplifier module is such that the system is characterised within awanted window of output powers for at least one intended application forthe RF transmitter module; and the training signal is defined by:z(t)=0.5(1+sin(ω₁t))exp(jω₂t).
 16. The method of claim 1, wherein themethod further comprises applying AM2PM pre-distortion to the trainingsignal comprising an envelope that varies with time.
 17. The method ofclaim 16, wherein the method further comprises refining the AM2PMpre-distortion applied to the training signal upon each iteration of:adjusting the mapping function between at least one instantaneousenvelope of a waveform signal to be amplified and the power amplifiermodule supply voltage to achieve a substantially constant poweramplifier module gain; and/or adjusting the timing alignment between theenvelope tracking path of the envelope tracking system and a transmitpath of the RF transmitter to align the envelope tracking poweramplifier module supply voltage to the at least one instantaneousenvelope of a waveform signal to be amplified.
 18. The method of claim1, wherein the at least one instantaneous output signal value comprisesat least one of: instantaneous power, instantaneous envelope,instantaneous phase.
 19. A non-transitory computer program productcomprising executable program code for calibrating an envelope trackingsystem for a supply voltage for a power amplifier module within a radiofrequency (RF) transmitter module of a wireless communication unit, theexecutable program code operable for, when executed at a communicationunit, performing the method of claim
 1. 20. A communication unitcomprising: a radio frequency (RF) transmitter module comprising anenvelope tracking system for a supply voltage for a power amplifiermodule within the RF transmitter module; and at least one signalprocessing module for calibrating envelope tracking system and arrangedto: apply a training signal comprising an envelope that varies with timeto an input of the RF transmitter module; receive at least an indicationof at least one instantaneous output signal value for the poweramplifier module in response to the training signal; and adjust a timingalignment between an envelope tracking path of the envelope trackingsystem and a transmit path of the RF transmitter module to align anenvelope tracking power amplifier module supply voltage to at least oneinstantaneous envelope of a waveform signal to be amplified by the poweramplifier module, based at least partly on the received at least anindication of the at least one instantaneous output signal value for thepower amplifier module in response to the training signal.
 21. Anintegrated circuit for a communication unit comprising a radio frequency(RF) transmitter module comprising an envelope tracking system for asupply voltage for a power amplifier module within the RF transmittermodule; wherein the integrated circuit comprises: at least one signalprocessing module for calibrating the envelope tracking system andarranged to: apply a training signal comprising an envelope that varieswith time to an input of the RF transmitter module; receive at least anindication of at least one instantaneous output signal value for thepower amplifier module in response to the training signal; and adjust atiming alignment between an envelope tracking path of the envelopetracking system and a transmit path of the RF transmitter module toalign an envelope tracking power amplifier module supply voltage to atleast one instantaneous envelope of a waveform signal to be amplified bythe power amplifier module, based at least partly on the received atleast an indication of the at least one instantaneous output signalvalue for the power amplifier module in response to the training signal.